This invention relates to a vehicular AC generator for use with a vehicular engine such as an automobile.
FIG. 1 illustrates a sectional view of a conventional vehicular AC generator, in which 1 indicates a front side bracket, 2 indicates a rear side bracket and 3 indicates a stator having a stationary core 3a and a stationary coil 3b wound thereon 1a and 2a indicate air inlet openings disposed in the end faces of the both brackets 1 and 2, 1b and 2b are air outlet openings disposed in an outer circumferential portion of brackets 1 and 2, and 20 indicates bolts for connecting open ends of the brackets 1 and 2 at the outer circumferential end portions of the stator core 3a to hold the core 3a therebetween. 6 indicates a shaft, which is rotatably supported by bearings 4 and 5 mounted to both the brackets 1 and 2 respectively. 7 and 8 indicate magnetic pole cores secured on the shaft 6 to position inside of the stator 3, 7a and 8a are pole pieces of the magnetic pole core, 9 indicates an excitation coil held between the inner circumferences of the magnetic pole cores 7 and 8, 30 and 40 indicate a pair of fans secured to the respective ends of the magnetic pole cores 7 and 8 for rotation with the shaft, 11 indicates a slip ring mounted to a the shaft 6, and 12 indicates a rotor composed of the shaft 6, the magnetic pole cores 7 and 8, the excitation coil 9, the fans 30 and 40 and the slip ring 11.
13 indicates an electric collector assembly having inside thereof a brush 13a contacting the slip ring 11, 14 indicates a rectifier which rectifies and converts an a.c. current, which is induced in the stator coil 3b when an excitation current is supplied to the excitation coil 9 from the brush 13a through the slip ring 11 and when the excitation coil 9 as well as the magnetic pole cores 7 and 8 are rotated by an unillustrated engine through a pulley 15, into a d.c. current, 14a indicates a heat sink for dissipating the heat generated in the rectifier 14, 16 indicates a voltage regulator for controlling the excitation current by detecting the generator voltage to regulate the terminal voltage, and 16a indicates a heat sink of the regulator 16. In the figures, L.sub.0 indicates a radial length dimension of blades 30a .about. 30e of the fan 30, L.sub.1 indicates a radial length dimension of the overlapping portions between the blades 30a.about.30e and the inlet opening 1a. Also, FIG. 2 illustrates a side view of the AC generator showing the positional relationship between the inlet opening 1a of the front side bracket 1 and the fan 30.
FIG. 3 illustrates a front view of the fan 30, in which 30a.about.30h are blades disposed at an equal angular interval .theta..degree., and in which the inner end of each of the blades is located at an equal distance from the center of rotation of the rotor 12.
FIG. 4 illustrates a front view of another fan 31 different from the above fan 30, in which 31a.about.31e are blades disposed at uneven angular intervals .theta..sub.1 .about..theta..sub.5 at every 180 degrees, and in which each blade 31a.about.31e has a curved portion which is tilted in the circumferential direction opposite to the direction of rotation toward the outer diameter.
The operation of the conventional AC generator will now be described. The rotation of the fans 30 and 40 causes cooling air to flow through the inlet opening 1a of the front bracket 1 as shown by an arrow a, to cool the bearing 4, the magnetic pole core 7, the excitation coil 9, the stator core 3a and the stator coil 3b, with the cooling air being exhausted to the exterior through the outlet openings 1b. Similarly, the cooling air is also introduced from the inlet opening 2a in the rear bracket 2 as shown by an arrow b, cools the bearing 5, the voltage regulator 16, the rectifier 14, the magnetic pole core 8, the excitation coil 9, the stator core 3a and the stator coil 3b, and exhausted to the exterior through the outlet opening 2b.
Since the conventional vehicular AC generator is constructed as described above, the radial length dimension L.sub.1 of the overlapping portion between the blades 30a.about.30e of the fan 30 and the inlet opening 1a is equal to or more than about 80% of the radial length dimension L.sub. of the blades as illustrated in FIG. 1, so that the portion of the radial length dimension L.sub.1 periodically and intermittently crosses the inlet opening 1a as the fan 30 rotates, resulting in a significant sound pressure of an annoying wind noise of a frequency of a rotational frequency multiplied by the number of the blades and a frequency of the number of the inlet openings 1a multiplied by an even number, i.e., the noise of 10th and 12th order frequencies in FIG. 2, leading to a main cause of an increased overall sound pressure level.
Also, when the number of the magnetic pole units 7a and the number of the blades of the fan 30 are equal as illustrated in FIG. 3 for example, the blades and the magnetic pole pieces mutually amplify when they rotate together, generating a wind noise of a frequency of a rotation frequency multiplied by the number of the blades and rotation frequency multiplied by the two-times the number of the blades, and thus, the wind noise of the frequencies of the 8th and the 16th harmonics becomes significant and annoying thus increasing overall sound pressure.
On the other hand, the fan 31 illustrated in FIG. 4 is the one improved from the fan 30, the main improvements being that the blades are disposed at uneven angular intervals at every 180 degrees, each blade has a curved configuration which is tilted in the circumferential direction opposite to the direction of rotation toward the outer diameter, and that the number of the blades and the number of the magnetic pole pieces are not equal to each other.
By applying the above improvements, with the fan 31 shown in FIG. 4, the overall level of the sound pressure level of the wind noise at various rotational speeds is generally decreased, but the ratio of the sound pressure of the frequencies of the 8th order and the 16th order to the overall level was not significantly changed. The reason for this is that two blades 31b of the fan 31 become small for the structural reason, so that it is not possible to match to the diameter of the inner ends of the other blades, whereby the effect of two blades 31b becomes small and has a substantially similar function as to that with eight blades. Thus, it has been very difficult to manufacture a fan in which the sound pressure level of the frequency of the orders of a multiple of the number of the magnetic pole pieces is decreased.